Romans came from the Italian pensinsula and came to Britain in the early first century. It was a gradual process and they slowly started conquering the different parts of the British isles during that time.
Vikings came from the Northern parts of Europe and they started conquering the British isles in later periods.
Both came and wanted to conquer the lands, while Romans were 'established' conquerors and started governing these areas soon by spreading their culture etc. The Vikings on the other hand had pillaing more in mind and did more of that and did not govern that much from the beginning.
The Tang Dynasty of China when faced with rebellion from General An Lushan in their northern territories sought outside support from mercenary forces. This was done a number of times during the rebellion, these mercenaries were typically Arab, however, we also have examples of soldiers brought from Uighur to fight with the Tang and numbered in the thousands in the fight against Lushan's forces.
With the help of mercenary forces, the Tang were able to take back several important capital cities of China and effectively eliminated the rebellion of Lushan, as well as his later supporters and successors.
The Empire State Building
